Generally, all contaminant removal scrubber towers include one or more spray levels that spray a reagent slurry onto the flue gas passing thereby. In most cases, this slurry is sprayed downwardly onto the counter-flowing flue gas so as to remove sulfur or other compounds from the flue gas as it flows upwardly through the tower. Of course, in other scrubber tower arrangements, the flue gas can be directed to flow downwardly or sideways through the tower which would thus necessitate the spraying of the reagent in an upwardly or a sideways manner. In any event, each spray level would generally consist of a multitude of spray nozzles that uniformly distribute the slurry about the cross section of the wet scrubber absorber tower through which the flue gas flows.
Usually, each such spray level is supplied its reagent slurry via a header arrangement that extends across the flue gas flow path. Such header arrangements oftentimes consist of a single main supply line and a multitude of individual feeder lines branching therefrom. These feeder lines further distribute or deliver the slurry to the spray nozzles, that are secured to these feeder lines, which atomize the slurry prior to being sprayed onto the flue gas. The number of nozzles and the nozzle locations are established so as to provide a uniform slurry coverage and also to provide the necessary slurry flow as required for sulfur or other contaminant removal criteria. Since practical limits exist on the flow capacity of each nozzle and the number of nozzles that can be used in a conventional spray header, multiple spray headers at various levels or elevations of the tower must be used so as to achieve the desired degree of contaminant removal. Furthermore, each spray level is supplied by an independent pumping system that itself contains redundancy so that operations may continue even though a portion of an individual pumping system may require maintenance.
An important consideration in the design of FGD scrubbers is the flue gas pressure drop through the scrubber tower. Pressure drop affects both the FGD system capital (i.e. construction) and operating costs. The wet scrubber spray headers cause a pressure drop simply by the fact that their presence in the flue gas flow path impedes such flow. Other mechanisms that affect the pressure drop in the tower include the spray nozzle pressure, flow rate, spray pattern, and nozzle spacing. Conventional spray header arrangements (i.e. one header with multiple feeder lines attached thereto) generally block approximately 22.5% of the total flue gas flow area within the tower.
Another important consideration in the design of FGD scrubbers is mist eliminator efficiency. Mist eliminators are typically chevron shaped inertial separators that remove small atomized droplets entrained in the upwardly flowing flue gas. The efficiency depends on the mist eliminator design details, flue gas velocities from the spray zone, and the quantity of mist created in the spray zone and flowing upward. This quantity of mist depends on the local gas velocities near the spray headers which depend on the flue gas flow area within the tower.
An advanced spray header arrangement recently developed by The Babcock & Wilcox Company is the interspacial spray header arrangement shown in U.S. Pat. No. 5,173,093 entitled "Single Spray Level for Flue Gas Desulfurization System." This spray header arrangement has typically two or more main supply headers located adjacent the wet scrubber tower at each level with multiple branches or feeder lines spanning across the flue gas flow path so as to supply slurry to the spray nozzles. The flow in each of these main headers is supplied by a separate pump and is roughly equivalent to the flow of a single conventional spray header. The advantage of this interspacial spray header arrangement is that it increases the number of spray nozzles and thus slurry flow at a particular level in the tower. It thus enables the tower to operate with fewer spray levels thereby reducing the height (and thus construction and operating costs) of the tower.
This interspacial spray header arrangement blocks approximately 45% of the total flue gas flow area within the tower. This increased flow blockage relative to a conventional spray header increases the flue gas pressure drop across the tower. This increased pressure drop offsets to some extent the cost savings achieved with the interspacial spray header arrangement. Also, the increased flow blockage increases the mist eliminator loading.
It is thus an object of this invention to improve upon both the conventional spray header arrangement and the interspacial spray header arrangement illustrated in U.S. Pat. No. 5,173,093. Another object of this invention is to provide a manner of increasing the slurry flow at a particular level within a scrubber tower without significantly increasing the blockage of the flue gas flow area by such a header arrangement. It is thus an object of this invention to increase the reagent slurry flow while only minimally increasing the pressure drop across the scrubber tower. Another object of this invention is to reduce the flue gas velocity in the vicinity of the spray header arrangement so that the sprayed and/or atomized slurry may be entrained for longer periods of time. Still another object of this invention is to reduce the volume of mist carried upward thereby reducing the amount of mist that must be eliminated (and also reduce its associated pressure drop) before the cleaned flue gas is discharged. Still another object of this invention is to provide for flow and atomization characteristics of the spray nozzles that are not compromised while still achieving uniform spray coverage. Yet another object of this invention is to reduce the scrubber tower height thereby reducing the capital and operating costs of such a tower.